LARVAE OF SAW-FLIES. 
517 
legs than true caterpillars, being provided with from eighteen 
to twenty-two ; hut their prop-legs have not the numerous 
little hooks that arm those of caterpillars. They have the 
means of spinning silk from their lower lips, hut not often 
in any great quantity. They are mostly naked and without 
hairs ; a few have forked prickles on their hacks ; some 
are covered with a white flaky substance, that easily rubs 
off; and others have a dark-colored slimy skin, which has 
caused them to he called slugs or slug-worms. They shed 
their skins about four times, and, after the last moulting, 
often materially change in appearance. Not only do these 
insects resemble caterpillars in their forms, but they have 
nearly the same habits. They are generally found on the 
leaves of plants, which they devour. Many kinds are al- 
together solitary ; a few live together in swarms, under silk- 
en webs, which they spin for a common place of shelter ; 
others are found also in swarms, but without any webs over 
them, and, when disturbed, they throw up their heads and 
tails, in a very odd way ; some roll up leaves, and live in 
the hollow thus formed, like the Tortrices; others make 
portable cases of bits of leaves, which they carry about on 
their backs, like the Tinese; certain kinds live within the 
stems of plants, and devour the pith ; and wheat, in Europe, 
is said to suffer considerable injury from internal feeders 
( Cephas pygmceus) of this kind. When fully grown, most 
of them go into the ground, and enclose themselves in thin 
silken cocoons, of an oblong oval shape, coated with grains 
of earth. Some make much thicker cocoons, in texture 
resembling parchment, and fasten them to the plants on 
which they live, or conceal them in crevices, or under leaves 
and stones on the ground. They generally remain for a 
long time unchanged in their cocoons, most of them during 
the winter ; are transformed to chrysalids, of a whitish color, 
in the spring, and come out in the winged form soon after- 
wards. Of some kinds there are two broods in the course 
of the summer, the false caterpillars of the first brood 
